Local positioning systems that employ a set of ground transmitters need to apply corrections for clock errors that are specific to the given transmitters in order to achieve precise positioning. There are various approaches to account for clock errors in order to achieve precise positioning.
According to one approach, at least one fixed base station is needed for implementing clock error corrections. The base station acts as a reference to provide differential phase measurements. The differential phase measurements are used for clock bias corrections for the ground transmitters. However, the use of a base station for implementing clock error corrections has certain disadvantages.
The location of the base station must be known precisely at a centimeter-level of accuracy. The precise relative or absolute positioning of the base station often requires surveying techniques that employ GPS Real Time Kinematic (RTK) solutions. However, a GPS RTK solution may not be possible at locations with poor or no access to GPS satellites. For example, a GPS RTK solution may not be achievable in a mining pit. Each time the base station is moved, another survey is required to determine the precise location of the base station. Further, the location of the base station must be such that the base station has a direct line of sight to any associated ground transmitters.
According to another approach, ground transmitter clocks can be synchronized to a common clock source in order to mitigate clock errors. An example of a common clock source is GPS time from a satellite. The use of GPS time assumes that the ground transmitters are capable of receiving GPS satellite signals, which assumption may not be valid. Moreover, the use of GPS time can result in unacceptable errors. In practice, the steering of the transmitter clock to a GPS time reference results in residual errors for both carrier-phase and code-phase signals. Such residual errors result in location solution errors that are in excess of GPS RTK accuracies, or 1/10 ns.
According to another approach, atomic clocks can be used in each ground transmitter. The clock drift of an atomic clock is negligible for purposes of precise positioning solutions. However, the expense and size of atomic clocks make atomic clocks impractical for low-cost, portable commercial applications.
Thus, in view of the above problems, there is a need for a method and system for implementing a precise positioning system at a centimeter-level of accuracy without resorting to the use of a base station, synchronization of transmitter clocks to GPS time, or the use of atomic clocks.